Fix playlist freeze: self-healing drag guard + identity-based row actions

A lost dragend (dragged node removed from the DOM) left dragFrom/cueDragging
stuck, permanently deferring renderPlaylist: the list stopped redrawing and
row buttons acted on stale indexes (delete/play appeared dead). The guard now
unlocks itself when no .dragging element exists, and row actions resolve the
track index at click time by object identity.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "dj-visualizer",
-  "version": "1.7.1",
+  "version": "1.7.2",
   "description": "Audio-reactive fractal & abstract visualizer for external display (Mac)",
   "main": "main.js",
   "author": {